Holographic optical data storage is an attractive alternative to magnetic tape, magnetic disc, and optical disc storage of digital computer data. It offers high capacity and high recording and reading data rates on storage media that can be removed from the drive, as described in Holographic Data Storage, H. J. Coufal, D. Psaltis, G. T. Sincerbox, editors, (Springer-Verlag, Berlin, 2000), incorporated herein by reference. Data to be stored is written to a photosensitive storage media by overlapping an information-bearing light beam (the signal beam) with a reference light beam. When the beams are coherent, coming for example from the same laser, standing waves in the beam's interference pattern create changes in the photosensitive material's index of refraction, thus forming a hologram. The stored data can be read out by illuminating the recorded hologram with the reference beam alone: the hologram diffracts light from the reference beam to create a copy of the original information-bearing beam. Multiple holograms can be recorded within the same volume of storage media by, for example, varying the angle of the reference beam. This is known as angular multiplexing. Many other hologram-multiplexing techniques are known in the art. The use of volumetric storage enables extremely high capacities, and the parallelism inherent in page-oriented storage offers much higher data rates that conventional serial bit-at-a-time technologies.
The information to be recorded or stored is imposed on the light beam through the use of a spatial light modulator (SLM). The SLM converts input electronic data to a two-dimensional image of bright and dark pixels, for example. Light modulated by the SLM passes through the optical system of the HDS device or drive to be recorded within the storage medium. In some instances, the SLM may modulate the phase (rather than the intensity or amplitude) of the light. Typically, a lens between the SLM and the recording medium is used to form a spatial Fourier transform of the SLM image in the region where the hologram is to be recorded in the photosensitive material of the storage medium.
Subsequently, when it is desired to read the data stored in the medium, the hologram stored in the recording medium is illuminated by the reference beam to reconstruct the SLM image, which can then be detected by a photodetector such as a CCD camera. One example of an SLM suitable for holographic data storage systems can be made using ferroelectric liquid crystals (FLCs) atop a CMOS backplane, constructed similarly to the microdisplay devices described in U.S. Pat. Nos. 5,748,164 and 5,808,800, the contents of which are incorporated herein by reference. These SLMs can be fabricated by techniques that are well known in the art, for example as described in “Semiconductor manufacturing techniques for ferroelectric liquid crystal microdisplays,” by Mark Handschy in Solid State Technology volume 43, pages 151-161 (2000), incorporated herein by reference.
However, several difficulties in the implementation of a practical holographic data storage system can be traced to the design and performance of the signal-beam optical path. Also, the particular FLC SLM devices described in the abovementioned patents do not make ideal write-heads. For example, when the SLM is operated as an intensity modulator, its Fourier transform contains a bright central spot, the DC spot, that is as much as 60 dB (one million times) brighter than the surrounding light intensity. This bright spot can saturate the optical recording medium, making it difficult to record and reconstruct data with high fidelity.
It is known in the art that the Fourier-plane DC bright-spot problem can be solved by introducing into the optical system a phase mask that imposes fixed, pseudo-random optical phase variations across the wave front. However, it is also recognized in the art [see, for example, U.S. Pat. No. 6,281,993, column 1 line 65 through column 2 line 4; or Maria-P. Bernal, Geoffrey W. Burr, Hans Coufal, John A. Hoffnagle, C. Michael Jefferson, Roger M. Macfarlane, Robert M. Shelby, and Manuel Quintanilla, “Experimental study of the effects of a six-level phase mask on a digital holographic system,” Applied Optics vol. 37, pp. 2094-2101 (1998), the contents of each of which are incorporated herein by reference] that the phase-mask image must be imaged onto the SLM, and that the phase-mask image must be very precisely aligned with the SLM (pixels in the mask must line up with pixels of the SLM image). The phase mask and its associated relay imaging optics adds to the size and cost of the drive's optical system, especially because of the precision optomechanics needed for micron scale alignment of the mask image to the SLM.
A crude attempt to eliminate the need for a separate relay lens to image the SLM onto the phase mask is described in the abovementioned book (Coufal, Psaltis, and Sincerbox, editors), in which Zhou, Mok and Psaltis disclose (p. 249) bonding a phase mask onto the exterior of a Kopin nematic liquid crystal SLM microdisplay to make an SLM for their holographic data storage system. They fabricated the phase mask as an array of lenslets having the same layout pitch as the pixel pitch of the SLM, each lenslet recessed into the substrate by a random choice of one of four different amounts, corresponding to the phase delays of 0, π/2, π, and 3π/2. The lenslet substrate was then bonded onto the outside of a completed transmissive liquid-crystal microdisplay.
In this design, the phase mask function is provided by the different recess depths. The function of the lenslets is to focus the light passing through the area of a given recess onto the corresponding pixel aperture. Since the construction of the nematic liquid crystal SLMs requires glass substrates nearly 1 mm thick, the recess relief pattern is at least this distance away from the plane of the pixel apertures. With desired pixel pitches in the range of 10-20 μm, the light passing through a pixel of the phase mask would, without the benefit of the lenslet, have undesirably spread to pass through many liquid crystal pixels by the time it had traversed the thickness of the SLM substrate. However, due to diffraction, the ability of small-diameter lenslets to focus light to a small spot on a plane a large distance away is limited. Usual Gaussian-beam optics dictate that the smallest spot the lenslet can focus a beam to has a diameter d=(4λ/nπ)(F/D), where F and D are respectively the focal length and diameter of a lenslet, λ is the optical wavelength, and n is the window's index of refraction. The lenslet diameter D could be no larger than the pixel pitch p, and to focus the light on the plane of the pixels, lenslet focal length F would be set equal to the thickness of the glass substrate. According to these relations, the thickest glass substrate that can be accommodated has a thickness F=(π/4)(np2/λ). Even closer spacing between the pixels and the lenslet plus phase mask than indicated by this equation may in fact be necessary to yield adequate image fidelity. For a typical SLM pixel pitch p=12 μm, and for light wavelength λ=0.5 μm, this approach of placing a phase mask on the outside of the SLM substrate of refractive index n=1.5 can only be effective if the substrate thickness is substantially less than 340 μm. It is impractical to industrially manufacture SLMs using glass substrates so thin and fragile.
It is against this background and with a desire to improve on the prior art that the present invention has been developed.